Continuous production of biodiesel in a packed-bed reactor using shell–core structural Ca(C3H7O3)2/CaCO3 catalyst

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Abstract

The continuous production of biodiesel was studied by using a steady-state packed-bed reactor. The shell–core Ca(C3H7O3)2/CaCO3 solid-base catalyst was prepared with a mechanical strong core of CaCO3 for continuous transesterification of soybean oil in a packed-bed reactor. Alcohol–oil ratio, retention time and reaction temperature were evaluated to obtain optimum reaction conditions. The yield of fatty acid methyl esters (FAME, i.e. biodiesel) achieved 95% at the reaction temperature 60 °C, alcohol–oil molar ratio of 30:1 and retention time of 168 min. The reusability of catalyst was checked up to 5 cycles and found negligible decrease in the catalyst activity. Water in the oil can significantly decrease the yield due to the deactivation of Ca(C3H7O3)2 and hydrolysis of FAME. The transesterification of soybean oil, canola oil and sunflower oil also was compared with model compound, triolein, using powder Ca(C3H7O3)2 in the batch reactor. Although these oils contained different triglyceride mixtures, their FAME yields were comparable. A Langmuir–Hinshelwood rate equation was established for the transesterification of soybean oil with methanol. Regression of experimental data indicated that the transesterification was an endothermic reaction with the enthalpy change of 23,504 J/mol and the activation energy was 42,096 J/mol.

Introduction

Biodiesel produced from vegetable oil and animal fats provides a good alternative to fossil fuel [1]. The Cetane number, flash point, and lubricity of biodiesel are better than those of fossil diesel [2]. Biodiesel sources do not contain significant amounts of nitrogen and sulfur compounds. Therefore, it has less amounts of NOx and SOx polluting emissions and much cleaner than fossil diesel fuel [3], [4], [5]. The simple alkyl esters of fatty acids, derived from oils also have uses other than as an energy source, such as in foods, textiles, cosmetics, rubber, and synthetic lubricant industries [6], [7], [8], [9], [10], [11]. The fatty acid methyl ester (FAME) is predominantly used as biodiesel.

Commonly, biodiesel has been manufactured by homogeneously catalyzed transesterification of vegetable oil with NaOH or KOH basic catalysts [12], [13], [14]. Base-catalyzed transesterification is much faster than acid-catalyzed. Although homogenous base catalysts have fast reaction rate under mild reacting condition, it is difficult to remove them from reaction mixture or products. In addition, a large amount of water is needed to wash them [15]. This washing process also is responsible for the waste products of stable emulsion formation and saponification. Homogeneous catalysts also are difficult to separate from glycerol generated as byproduct [16], [17].

Solid-base catalysts have great potential for biodiesel processing with reasonable reaction rates under mild conditions. Alumina-supported alkali elements/hydroxides, MgO, CaO, ZrO2, calcined MgAl hydrotalcites, rehydrated hydrotalcites, anion-exchange resin and alkali-exchanged zeolite were used to study biodiesel synthesis utilizing heterogeneous catalytic processes [12], [18], [19], [20], [21], [22], [23], [24], [25], [26].

Recently, much interest has been taken in CaO due to its economy and reactivity for the transesterification of soybean oil and poultry fat [21], [27], [28]. Some researchers observed that the CaO slightly dissolved in methanol during transesterification process. On the other hand, CaO was transformed into calcium diglyceride by combining with glycerol during transesterification of soybean oil with methanol [28], [29]. The leaching of the solid-base catalyst occurred simultaneously with the active phase transformation. The sensitivity of CaO to atmospheric CO2 to form CaCO3 is the major disadvantage because CaCO3 is inactive for transesterification reactions. The solid phase Ca(C3H7O3)2 catalyst has overcome CaO deactivation and separation problem [29]. However, the mechanical strength of CaO particle is weak so it would collapse in a packed-bed reactor. On the other hand, CaCO3 particle is mechanically strong to be used inside a packed-bed reactor.

The objective of this study was to demonstrate a continuous biodiesel production process. A shell–core Ca(C3H7O3)2/CaCO3 chunk with a mechanically strong CaCO3 core can facilitate low-pressure drop in packed-bed reactor. The transesterifications of soybean oil, canola oil, sunflower oil and model compound, triolein, were compared to reveal any effects of different triglyceride mixtures. The influence of retention time, temperature and methanol-to-oil ratio, was investigated to achieve the optimized reaction conditions. Finally a Langmuir–Hinshelwood model also was established to correlate the experimental result.

Section snippets

Materials and characterization

Soybean oil, canola oil and sunflower oil (all FFA contents <0.5%) were obtained from Uni-President Company, Taiwan and used directly without pretreatment. CaO (98%) and 2-propanol (>99.8%) were purchased from Sigma–Aldrich. Triolein (C57H104O6 > 97%) and chemical grade methanol (>99.8%) were purchased from Fluka.

Shell–core CaO/CaCO3 catalyst was prepared by calcining CaCO3 chunks (5–6 mesh size, ∼4 mm) in a helium atmosphere at 900 °C for 1.5 h at heating rate of 6 °C/min. An outer layer CaO was

Characteristics of catalyst

The calcination of the CaCO3 powder and chunks gave CaO and CaO/CaCO3 respectively, at 900 °C under helium atmosphere. The CaO, on reaction with glycerol, yielded Ca(C3H7O3)2 [29], [30]. The Ca(C3H7O3)2/CaCO3 chunks contained thin layers of Ca(C3H7O3)2 on their surface. That Ca(C3H7O3)2 layer was carefully peeled off and ground for XRD analysis as shown in Fig. 2. Calcium diglyceride formation on Ca(C3H7O3)2/CaCO3 catalyst was observed in Fig. 2(a) with the presence of characteristic peaks of

Conclusions

A Ca(C3H7O3)2 catalyst was synthesized and used for the batch transesterification of variety of oils, included canola and sunflower oils. Soybean oil gave the maximum transesterification yield due to short carbon chain of fatty acid. A shell–core Ca(C3H7O3)2/CaCO3 catalyst was successfully synthesized by the calcination of CaCO3 particle followed by glyceride reaction. This catalyst was employed in a continuous flow packed-bed system, in which the products and the catalyst could be separated

Acknowledgement

Financial support by the Ministry of Economic Affairs, Taiwan, under grant 97-EC-17-A-09-S1-019 is gratefully acknowledged.

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